I have never changed the cables on a CAAD12 but have done so on at least four Cannondale Synapes in the last year. The Synapse has a pug where the back end of the cables exits the tube. With the plug out I just feed the cable from the front while shining a light in the back hole. Once I see the cable end I hook it with a dental pick (a bent paper clip would work) and fish it out. It takes no time at all. I am not familiar with the CAAD12 so maybe it is different. It has worked on other bikes too.
This does not work on my Santa Cruz Hightower but I had enough room to cut the head off the old cable near the shifter and tape the tail of the new cable to it and fish it through. I was careful to mostly push on the new cable and lightly pull on the old one to guide it as I did not want to pull the taped connection apart.
Hope this helps.